1. Technical Field
The present invention relates to the transmission of digital signals via digital microwave radio. More particularly, the present invention relates to measuring the susceptibility of digital microwave radios to multipath fading.
2. Description of The Prior Art
A valuable way of assessing the performance of a digital microwave radio (`DMR`) with regard to a propagated microwave signal is to measure the DMR receiver's susceptibility to multipath fading. In FIG. 1, a transmitter 10 is shown sending a signal to a receiver 12 over multiple signal paths, variously shown as a direct and intended path 15, and as a reflected path 14 and a ground bounce path 16. Due to the differences in distance the various signals travel over the multiple paths, they arrive at the receiver 12 at slightly different times. Thus, there is often signal fade due to phase difference cancellation between the signals. Testing susceptibility to multipath fading is particularly important since fading is recognized as one of the predominant causes of unacceptable data bit error rates and link outages.
The DMR's susceptibility to multipath fading is measured using the M-curve measurement which was developed by the British telecommunications industry. The M-curve is a measure of the depth of a notch at a particular position in the frequency band of a DMR which causes the DMR to experience a given bit error rate (`BER`).
The M-curve measurement is typically made as follows: The DMR transmitter IF output is coupled to the input of a multipath fading simulator, such as the HP 11757B manufactured by Hewlett-Packard Company of Palo Alto, Calif. The output of the multipath fading simulator is coupled to the IF input of the DMR receiver. The BER is monitored while the transmit and receive paths of the DMR are exercised.
When no fade is occurring, the BER is zero. For any point to be plotted on the M-curve, a notch is imposed in the IF band of the DMR at a specific frequency, while the BER is monitored. The notch is slowly increased in depth until a specific BER is reached, for example 0.0001. The same measurement is repeated at different frequency points within the IF of the DMR. When the resulting measurements of notch depth versus frequency are plotted the resulting curve gives an indication of the DMR's susceptibility to multipath fading.
Normally, data provided to a DMR are converted in a DMR transmitter to `In-phase` or `I` signals and `Quadrature` or `Q` signals. These signals are used to modulate a baseband carrier having a typical frequency of 70 MHz or 140 MHz. The modulated carrier is then up-converted to a much higher frequency and transmitted over a signal path.
At the receiving end of the signal path, a DMR receiver down-converts the high frequency `IQ` modulated signal to the 70 MHz of 140 MHz baseband signal, demodulates the `I` and `Q` signals, and converts the `I` and `Q` signals back into digital data.
In the DMR transmitter, the `I` and `Q` signals each go through a separate filter. In the DMR receiver, the demodulated `I` and `Q` signals also each go through a separate filter. The purpose of the filters is to provide an overall transmit-to-receive characteristic which minimizes intersymbol interference.
The receiver in a DMR functions such that the `I` transmitted signal can be received by either the `I` or the `Q` receive filter. Similarly, the `Q` transmitted signal can be received by either the `Q` or the `I` receive filter. Accordingly, there are two different sets of transfer characteristics possible for a propagated signal. The two sets of transfer characteristics (two transmit and two receive filter characteristics) may be expressed as follows:
HITR(S)&gt;`I` transmit filter frequency response; PA1 HQTR(S)&gt;`Q` transmit filter frequency response; PA1 HIRC(S)&gt;`I` receive filter frequency response; and PA1 HQRC(S)&gt;`Q` receive filter frequency response. PA1 Set 1: HITR(S)*HIRC(S), HQTR(S)*HQRC(S) PA1 Set 2: HITR(S)*HQRC(S), HQTR(S)*HIRC(S).
Thus, the two sets of overall transfer characteristics are:
Ideally, HITR(S), HIRC(S), HQTR(S), and HQRC(S) are all equal. However, it is both difficult and expensive to produce filters having identical transfer characteristics. Thus, in practice the filters are not identical or perfect, and the two sets of transfer characteristics each have a slightly different effect on intersymbol interference.
The two sets of transfer characteristics are referred to as lock states wherein the DMR `locks` up with `I`--`I`, `Q`--`Q` or `I`-`Q`, `Q`-`I` for the transmit-to-receive signal path. FIG. 2 is an x-y graph of six M-curves plotting notch depth in decibels versus notch frequency in MHz. The M-curves shown were measured without regard to the different lock states. The numbers on each curve indicate the number of data points falling on the curve at that frequency. The two curves show the two different lock states which occured during the data acquisition process. The distance between the curves is an indication of the inaccuracy of known techniques for establishing the M-curve.
It is not possible to force the DMR to select the best set of transfer characteristics. Accordingly, it is desirable to measure DMR sensitivity on the worst set of transfer characteristics. The M-curve measurement described above does not provide for using worst case lock state.